Sublines of transplantable rat adenocarcinoma MTW9 have been developed which differ in content of estradiol and prolactin receptors, ovariectomy response, and growth rate. These will be studied in serial transplant and in culture for genetic stability, and growth control by hormones. The regulation of receptor content and nuclear transport of these sublines will be studied in terms of hormonal environment in an effort to explain why some tumors that possess estradiol receptor do not regress after ovariectomy. We have developed techniques which may distinguish receptor sites which are apparently homogeneous by Scatchard analysis. These techniques will be utilized in measuring receptor sites for estradiol and prolactin in tumors growing under different hormonal conditions. MTW9 grows in only 20% of hosts with a normal serum prolactin but increased environmental temperature increases the number of tumors which grow out. This phenomenon is under further study. MTW9-MtT (MTW9 growing in a rat with co-implanted mammosomatotropic MtT does not regress after ovariectomy). The factor inhibiting ovariectomy regression is not prolactin or growth hormone. We will attempt to identify it.